US2363650A

US2363650A - Frequency modulation detector circuit

Info

Publication number: US2363650A
Application number: US450355A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-07-10
Filing date: 1942-07-10
Publication date: 1944-11-28
Anticipated expiration: 1961-11-28

Description

Nov. 28, 1944. M G. CROSBY 2,363,650

FREQUENCY MODULATION DETECTOR CIRCUIT Filed July 1o, 1.942 2 sheets-sheet 2 T 1:1.E..a. 1:1 Eld [E/M/JL/F/ff? l l dk4 c rfv/r y iss @N4/vrs @f2/,Wis

ATToRNEY Patented Nov. 28, 1944 ztatoK FREQUECY Mcitfmlq HECTOR j. i

`Murray G. Crosby,Rlverhea`d,

` Radio Corporation of America,

of Delaware N. Y., assigner to a corporation Animation July 1o, 1942, sierieiNo. 450,355

` ilcieims. V(ol. 2511-1-27) My present invention relatesto frequency modulation receivers, `and more particularly to an improved method of, and meansfor, reducing the effects of `amplitude `modulation, introduced by virtue; of` round-topped resonance curve characteristics of signal selector circuits."

In the past it has beenproposed to eliminate the amplitude modulation limiter :stage from receivers of frequency modulated carrier wave energy (FM) Whenthe limiter is removed from such a receiving. system, say one ofthe `universally-used nsuperheterodyne type, one of the `ef- `fects encountered isthe introduction of amplitude modulation due to the `variations of `frequen-cy engendered by virtue of the vround-toppedv selectivity curve of theintermediate frequency (I. F.) amplifier which precedes the usual dis-` criminator-rectier network. .l -Such amplitude `modulation shows up as a distortion in the receiver output. `.Aside from the use of `a limiter stage to remove amplitude` modulation, various Y inversemodulation circuits have been proposed and described in my application Serial No.`

416,443, filed. Oetoberf25fl94l, and in my application Serial No.` 435,467, filed March 20, 1942.

, It may be stated that it is `oneof 1` the main objects of my presentinvention to provide a rel ceiver `of angular velocity modulated carrier wave energy wherein the effect of amplitude modula-` tion. is minimized byladjusting the discriml'nator characteristic so that compensating distortionis introduced which .cancelslfthe undesired ampli tude modulationCAM).

Another important object of this invention is to provide a discriminator circuit for a frequency modulation` receiver, `wherein. a compensating distortion is applied to the discriminator. charac.`

\ teristic `so that the `resulting overall discriminator characteristic is linear, f Another `object of the invention is to reduce the effects of amplitude modulation, introduced by a round-topped I..F.,selec.tivity characteristic, in afrequency modulation receiver of the type which does not employ a limiter stage per se,

Stillother objects of the invention'are to improvegenerally the simplicity and'teffciency of FM receivers, andmore .especially to provide FM receivers of the superheterodyne type, free of any limiter stage per se,- which are; reliable in operation and `are `economically manufactured and assembled. The novel featureswhich I believe to be char-M aeteristic of my invention` are set `forth with partioularity in the, appended claims; the invention itself; however.. as1 to both its organization and method. of operation `will best be understood.` to the following description taken` with the drawings inwhich I have by reference in connection indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows a circuit arrangement of a dise.

criminator-rectier network `of` an `FM receiver embodying the invention, u

Figs. 2ato 2f inclusive show various char-acteristics o-f the invention.`

Referring now to Fig."1,`t`here`is `shown here that portion of an FM `receiver of` the superheterodyne type located between an early I. F.; amplifier and themodulation voltage utilization network. Assuming that the receiveris operated in the presently assigned FM bandof 42-50 megatothe center frequency plied at I ultra-high cycles (me), those skilled in:` the art` are` fully acquainted with the fact FM channel has alpermissible deviation width4 ofl 200 kilocycles (kc.) .l mitter the carrier frequency is` deviated over a range of of this `application to point out that at the transmitter the carrier frequency is deviated vback `and `forth with respect to a center or mean frequency KFC) in accordancewith modulation signal ampli` The rate of frequency deviation depends z The j tude.` upon the modulation frequencies per se. generic expression angular velocity-modulated carrierwave energy" is tobe understood as cov` ering either frequency modulated, or phase modulated, carrier Wave energy.

IntheFM receiver? u u frequency in one or more amplification stages whichhave been adjusted ofthe particular desired FM channel. The amplified FM energy -is then reduced in center frequency to the LF. value. The latter may bechosen me-; by way of `illustration it can be assumed that the I F. value is of the order'of 4.3 mc. Of course, the frequency reduction of the center `frequency `to I. F. value does not affect the devia; tion range of the FM energy. Hence, bothprior to the converter and after the converter the se` lector circuits have a pass band width of the order of 200 kc.,.or at least in excess of the overall deviation range of the FM energy.`

Considering Fig. 1 specifically, itbeassumed that amplifier which feeds the discriminator network of `the FM detection circuit; The I. F. amplifier may be -of usual construction.' 4Its signal grid l' "connected to a prior therefore, iet

iS to be understood as being that in this band each.

Actually, at the trans-` 150 kc. It is sufficient for the purposes u the radiated energy is am-` from a range of 2 to 20` numeral l designates an I. F.`

`6. The detected FM I. F. amplifier stage (or, if desired, to the I. F. output circuit of the converter stage). The I. F.- tuned transformer T couples the prio-r amplifier to tube I.' The plate I 4 of tube I is connected to a point of proper positive potential through series-arranged coils 2 and 3. Coil 2 is shunted by tuning condenser 2*,'while coil 3 is shunted by tuning condenser 3. Each of coils 2 and 3 is, moreover, shunted by a respective damping resistor I2 and I3. The cathode circuit of amplifier tube I includes the usual bypassed bias resistor I.

The circuits 2-2 and 3 3 provide the usual discriminator network for transforming the FM energy into AM energy. Those skilledin the art are fully acquainted with the functioning and structure of the discriminator network. Circuit 2 2' is tuned to a frequency spaced from the center frequency by a predetermined frequency* I amount. Circuit 3 3' is tuned to a frequency spaced from the` center frequency by the same frequency amount, but on the other side of center or mean frequency. It is desired that the frequency spacings between the resonant peaks of the two discriminator 'circuits exceed the overall deviation range of the center frequency. Of course, the latter value would be the operating I. F. value in the present instance.

It will be noted that no limiter stage per se is employed between the I. F. amplifier and the discriminator circuits. Those skilled in the art are fully aware of the function of a limiter network. It acts to eliminate amplitude modulation effects which may develop on the carrier. Such amplitude modulation effects are created by fading, extraneous noise, and because of the round-y topped selectivity curveof the various selector circuits prior. to the discriminator network. In my aforementioned applications, I have shown i various circuits for-eliminating AM effects on theFM energy without using a limiter per se. It is desirable to utilize a simple method of eliminating such'AM effects, so that the detected output voltage will be a function solely of the frequency deviation of the carrier and not produce harmonic distortion by virtue of the carrier am plitude variation.

The discriminator functions to convert the frequency-variable, constant-amplitude carrier energy into carrier energy whose amplitude is variable in accordance with the modulation, but whose center frequency remains invariable. The discriminator output energy is subjected to rectication in the manner shown in Fig. 1. For example, a pair of rectifier diodes 4 and 5 have their cathodes arranged in series with the load resistors 8 and 9. The anode 4 is connected by coupling condenser 20 to the plate side of discriminator circuit 2 2. The junction of resistors 8 and 9 is connected to the junction of a pair of resistors between anodes 4 sistors 6 and 1 denser 2| to the 6 and 1. arranged in series and 5. The junction of reis connected by coupling condjunction of coils 2 and 3. The

- anode end of resistor 1 is grounded.

Each of resistors 8 and 9 is shunted by an I. F. bypass condenser, and these bypass condensers are respectively designated by the numerals 8' and 9. The modulation signal voltage corresponding to the FM component of the received waves is taken off from. the anode end of resistor voltage is derived from the rectified voltages developed across each of resistors 6 and 1; these rectified voltages are in polarity opposition relative to the grounded end of reiI5 tuned circuits 2 2' and 3 3.

sistor 1. Detected AM signal voltage may be taken off from the cathode end of resistor 8. The detected AM voltage is derived from the rectified voltages developed across resistors 8 and 1; these rectified voltages are in additive phase relative to -the grounded end of resistor 1.

In explaining the functioning of the present invention, attention is first directed to the typical round-topped I. F. selectivity characteristic shown in Fig. 2a. This is assumed to be the selectivity curve of the I. F. amplifier up to tube I.

It 'is possible to obtain a more ideal flat-topped characteristic, but usually the expense goes up as this is done. Furthermore, such a more ideal characteristic isdiincult to augn, and is name to drift. The actual discriminator characteristic of. Fig. 2b is that proposed by my present invention. It `will be noted that the characteristic is straight, or linear, in the middle portion Where the I. F. characteristic is relatively fiat. In other words,'the middle linear portion of the curve of Fig. 2b corresponds to the relatively fiat portion of the characteristic of Fig. 2a. In the portions of the characteristic of Fig. 2a where 'drooping commences, the discriminator characteristic rises a compensating. amount so that the overall characteristic is linear as shown in Fig. 2c.

I have found that a discriminator characteristic such as is shown in Fig. 2b maybe prodduced with the circuit shown in Fig. 1. f, As stated heretofore, the discriminator is composed of off- These discrimif nator circuits are off-tuned to either side of the center frequency, so that `their individual resonance curve characteristics are as shown, for example, by the curves A and B of Fig. 2d. When the discriminator network is properly adjusted, an overall effective linear discriminator characteristic may be obtained as shown in Fig. 2c. This may be donel by adjusting the inductive coupling between coils 2 and 3 to be equal to the capacitive coupling between the off-tuned circuits. The reference character M denotes the inductive` coupling, `while the dotted capacityC denotes the capacitive coupling between the discriminator circuits.

In addition, the damping resistors I 2 and I3 are adjusted properly to apportion the selectivity 4of the discriminator circuits. It is pointed out that the linear discriminator Fig.A 2c would be desirable if a limiter stage were used prior to the discriminator network. A simple way to provide the .characteristic of Fig. 2b is as follows: ChooseM, C, I2 and I3 to get the curve of Fig. 2c, and then raise the values of I2 and I3 to produce the curveof Fig. 2b. For this adjustment the damping resistors I2 and I3 are made higher in resistance, so that the tuned circuits are more sharply selective. The result is the characteristic of 2b, which has the desired compensating distortion.

' The compensating distortion of the discriminator characteristic is equally effective on amplitude modulation reception with the same detecting system. Assume, now, that AM carrier waves are being received by the same receiver, and that the latter has been adjusted to receive such Waves. In this case the detected outputs are added, instead of subtracted, so that a double-peaked characteristic is secured, as shown in Fig. 2e.. This curve is taken at the upper end of resistor 8 assuming omission of the round-topped curve of' Fig. 2a. Increasing the value of the damping resistors has the effect of' making this double#- peaked characteristic more marked. The overall characteristic of ampliiied Iin 'the usual way. I and il show resistor-condenser filter networks used respectivelyn the FM and AM take-off leads result of the round-topped I. F. characteristic of Fig. 2a and the double-peaked characteristic of Fig. 2e is the flat-topped characteristic shown in Fig. 2j. It is assumedin this case, that the I. F. amplifier feedingthe detector has the charac- `thereof teristic of Fig. 2a. Detected AM voltage, taken off fromthecathode-end of resistor 8, may be The networks I0 to remove carrier energyffrom the output circuits. While `I have indicated and described a system for carrying my invention into effect, itlwill be apparent toone skilled in theart that my invention is by no means limited to the particular organization shown and` described, but that many modifications may be made without departing effects,

central region a suiiicient amount substantially to compensate forthe said drooping flanks thereby to compensate for said amplitude modulation and means connecting said rectifiers in i polarity opposition to produce a modulation signal voltage corresponding to thefrequency modulation of said carrier waves.

3. In combination, in a receiverof frequencym Y modulated carrier waves; a detector provided with from the scope of my invention, as set forthin f v the appended claims.

What I claim is: i l l. In combination, in a receiver of angular vvelocity-modulated carrier waves, a detector provided with` a discriminator network', a selective ampliiiercoupled to said discriminator to apply amplified modulated waves to, the latter, said amplifier being tuned :to a predetermined referencefrequency and having a frequency vs. arnplitude characteristic which possesses substantially` drooping flanks relative to said reference f frequency whereby amplitude modulation effects are introduced on said carrierwaves, said discriminator s s equal tosaid reference frequency and being con` being tuned to a center frequency,"

structed and arranged to have a frequency vs.

direct current output `characteristic whose slope is relatively linear over the median region thereof but rises in opposite directions from the median region a sufficient amount substantially to `com-` pensate for the said droopingiianks therebyto, compensate for said amplitude modulation effects.`

2.In combination, in a receiver `offrequency modulated carrier waves, a detector comprising a pair of rectiiiers, a frequency discriminator network, a` selective amplier coupledto said dis-` criminator networkto apply amplified frequency modulated waves to the latter, said amplifier be` ing tuned to a predeterminedfrequency and having a frequency vs. amplitude"characteristic which possesses substantially drooping` flanks whereby amplitudemodulation effects` areintroduced on said `carrier waves, `said discriminator network and arranged to have `a frequency vs. direct current output characteristic"whose` slope is relabeingtuned `to` a frequency equal to said i `predetermined frequency and being constructed a frequency discriminator network, a selective amplifier coupled` to said discrminator to apply frequency modulated waves to the latter, said amplifier being tuned to a predetermined operatingfrequency and having a frequency vs. amplis `tude characteristic'which possesses a substantially round top and substantially drooping flanks,` i whereby amplitude modulation effects are intro.-

duced on said carrier waves, said discriminator `network comprising a pair of resonant circuits tuned to respective frequencies located on oppo- ,s site sides of said operating frequency, said pair of resonant circuits being magnetically and ca` pacitatively coupled to provide afrequency vs. direct current output discriminator characteristic i whose slope is relatively linear over the median region thereof but, rises in opposite` directions from the median region a sufficient amount sub- "stantially `to compensate` for the said drooping flanks thereby to compensate for said amplitude `modulation effects. q

4. In combination, 1n a receiver of frequency` modulated carrier waves, `a detector provided with a frequency discriminator network, a `frequency selective circuit coupled to said discriminator network to applyfrequency modulated `waves to the s latter, said selective circuit being tuned to a pre determined reference frequency and having a frequency vs. output amplitude characteristic which possesses substantially drooping iianks relative to q said reference frequency whereby amplitude modi ulation effectsare introduced on said carrier waves, said discriminator network being tuned to a center frequency equal` to said refer-encefrequency andbeing constructed and arranged `to `have a frequencyvs. direct current output characteristic whose slope is relatively linear over the median region thereof but -rises in opposite direcw region a sufficient amount `substantially to compensate for the said drooping flanks thereby to` compensate `for said amplitude tions from the median modulation effects.

i MURRAY G. CROSBY.

. 3 tively linear over a predetermined centralregion but rises inopposite directions from said